Remove the unused DatasetVersionFileTreeManager

### Task Summary

`DatasetVersionFileTreeManager` builds a client-side hash map of dataset file paths. Nothing in production constructs it; its only remaining references were its own declaration and its spec block.

### History

| | |
| --- | --- |
| **Introduced by** | #2413 (2024-02-26) — "Introduce Dataset GUI" |
| **Usage removed by** | #3296 (2025-03-10) — "Add FileService as a standalone microservice, LakeFS+S3 as dataset storage" deleted the real usages from `files-uploader.component.ts` (`previouslyUploadFilesManager`, `newUploadFileTreeManager`), since LakeFS now serves the file tree. The leftover dangling import was swept away by #3848 (2025-10-11, the `org.apache` rename) |

Dead for about a year and a half. The rest of `datasetVersionFileTree.ts` stays — `DatasetFileNode` and `getFullPathFromDatasetFileNode` have live consumers.

Pure deletion, no behaviour change: −287 lines.

### Task Type

- [x] Refactor / Cleanup